The present invention relates to an image sensing apparatus suitable for using a sequential scanning type solid-state image sensing device covered with a color filter of so-called Bayer arrangement and the like.
Recently, an image sensing device, such as CCD, capable of sequentially reading signals of all the pixels (referred as "non-interlace scanning type image sensing device", hereinafter) has been developed with the progress of semiconductor manufacturing technique. The non-interlace scanning type image sensing device has an advantage in that a higher resolution image can be obtained with less blurring than an image sensed by using a conventional interlace scanning type image sensing device even when sensing a moving object. In the interlace scanning type image sensing device, a frame image is composed of two field images which are sensed at different times, usually at a field period interval. Accordingly, there is a problem in which, when sensing a moving object, there are notches on edges of the object and perhaps in the background in a frame image because of the time gap between the two field images composing the frame image. If a frame image is made of image data of a single field image to overcome the aforesaid problem, there would not be notches on edges, however, the vertical resolution of the obtained frame image is halved.
A conventional image sensing apparatus using an image sensing device which outputs signals after adding two vertically adjacent pixel charges will be explained with reference to FIG. 16. Referring to FIG. 16, an image sensing device 101 outputs signals after adding two vertically adjacent pixel charges in accordance with timing signals t'1 and t'2 generated by a timing signal generator (TG) 109. The output image signals are inputted to a correlated double sampling (CDS) circuit 103 via a buffer 102, and reset noises of the image sensing device 101 are removed from the output image signals by the CDS circuit 103, then the image signals enter an automatic gain controller (AGC) 104. In the AGC 104, the image signals are amplified by a gain set in accordance with a control signal c'2 from a microcomputer 108 (gain control). The gain-controlled image signals are converted into digital signals by an analog-digital (A/D) converter 105, then transmitted to a camera processing circuit 106, where predetermined processes are applied to the digital image signals, and a luminance signal Y and a color difference signal C are outputted. Further, the microcomputer 108 generates a control signal c'2 for controlling the gain in the AGC 104 in accordance with the gain information c'1 detected by a camera processing circuit 106.
In contrast, with a non-interlace scanning type image sensing device, it is possible to sense a frame image in a field period, thus, the aforesaid problems do not arise. Accordingly, the non-interlace scanning type image sensing device is expected to be applied to a still image camera and a camera for EDTVII (Extended Definition Television II), for example.
A circuit configuration of an image sensing apparatus using the conventional non-interlace scanning type image sensing device which reads out signals by two horizontal lines will be explained with reference to FIG. 17. A non-interlace scanning type image sensing device 201p outputs image data of one frame in a field period, thus the speed for transferring charges is two times faster than the transferring speed of an image sensing device, as shown in FIG. 16, which outputs image signals obtained by adding two vertically adjacent pixel charges. Accordingly, it is preferred to design an image sensing device to have two horizontal registers which respectively transfer image signals of odd and even scan lines simultaneously to be first and second output signals, instead of transferring by one line through a single horizontal register.
It should be noted that most of the non-interlace scanning type image sensing devices used at the present time are provided with R, G and B filter chips arranged in so-called Bayer arrangement as shown in FIG. 2. In this color arrangement, G signal is used as a luminance signal.
The non-interlace scanning type image sensing device 201p transfers charges via vertical registers and horizontal registers in accordance with timing signals t1p and t2p generated by TG 209p, and signals on the odd lines and the even lines are outputted from the channels ch1 and ch2 in each field period. The output signals from the channels ch1 and ch2 are respectively sent to buffers 221 and 222, then to CDS circuits 231 and 232. The CDS circuits 231 and 232 remove reset noises of the image sensing device 201p, then transmits image signals to AGCs 241 and 242. The AGCs 241 and 242 amplify the image signals by gains which are designated by the camera processing circuit 206p. A/D converters 251 and 252 convert the analog image signals into digital signals, then transmits them to the camera processing circuit 206p.
Among the digital image signals inputted to the camera processing circuit 206p, it interpolates the G signal, used as a luminance signal, in the horizontal direction as shown in FIG. 18A so that all the pixels have the luminance data. Thereafter, gain information c1p obtained on the basis of the G signal is sent to a microcomputer 208p. The microcomputer 208p determines gains to be used in the AGCs 241 and 242 on the basis of the data received from the camera processing circuit 206p, then sends a control signal c2p.
The AGCs change gains to be applied to image signals in accordance with the control signal c2p, and the two AGCs have different characteristics from each other in general. Therefore, even though the same gain is provided to the two AGCs, the levels of amplified image signals may differ from each other. If this occurs, when an object of a uniform color (e.g., a white paper) is sensed, a variation in output signal level of the two AGCs appears in a stripe pattern of odd and even scan lines as shown in FIG. 18B. Therefore, when an image of the object is displayed, the variation in output signal level appears as the difference in output signal level between odd and even line fields on a display, which causes field flicker. This noticeably deteriorates the quality of the image.
To overcome this problem, a method for interpolating an average of pixel values of the G signals in a vertical row as shown in FIG. 19A can be considered. However, in this method, when an object of a uniform color (e.g., a white paper) is sensed, a variation in output signal level of the two AGCs may cause vertical stripes which alternatively have different output levels on a display as shown in FIG. 19B. The difference in output level in the vertical stripes also noticeably deteriorates the quality of an image.
Further, in order to compensate a variation in output signal level of the two AGCs, the following method may be considered. First, the TG 209p sends a timing signal t3p to a test signal generator 210p at a predetermined timing. The test signal generator 210p inputs a test signal to the two horizontal registers ch1 and ch2 of the image sensing device at a timing in accordance with the timing signal t3p. The test signals outputted from the two horizontal registers are processed in the same manner of processing image signals. Then, the difference in output level between the two AGCs is detected and the microcomputer 208p sends gain information c2p to the AGCs 251 and 252 based on the gain information c1p which shows gain difference between the test signals. Further, the microcomputer 208p outputs a signal d1 which instructs to start the next test to the test signal generator 210p, in turn, a test signal is inputted to the horizontal registers again and processed in the same manner of processing image signals so as to confirm that the difference in output level between two AGCs are corrected.
However, in the configuration shown in FIG. 17, means for generating a test signal used for correcting levels of signals output from the two AGC circuits, increases the size of the apparatus. Further, since it is necessary to confirm that the output signals from the AGCs are successfully corrected, it consumes a considerable time for the confirmation.